1. Field of the Invention
The present invention relates to the field of automobile wheels and, more particularly, to an improved automobile wheel rim and method of making same.
2. Description of the Prior Art
Automobile owners often desire to improve the appearance and performance of their cars by replacing the original equipment wheels supplied by the vehicle manufacturer (OEM) with aftermarket wheels of lighter weight and more stylish appearance. While many owners are satisfied with aftermarket wheels that are comparatively more stylish than OEM wheels, some owners, xe2x80x9cautomotive enthusiastsxe2x80x9d (drivers of sports and high performance type cars), desire wheels that are of substantially lighter weight and therefore improve the performance as well as the appearance of their vehicles.
Automobile wheels of a type which provide a decorative appearance and relatively light weight are most commonly manufactured by using a light weight casting alloy, typically A356 or A357 aluminum, to cast a wheel in the desired configuration. Typically, these wheels are cast as one-piece units with integral wheel-centers and rims. After casting, the wheels are finish machined to final dimensions. There are many examples of wheels of this type, one of which is U.S. Pat. No. 5,292,182, entitled xe2x80x9cLight Alloy Cast Wheelxe2x80x9d issued to Kanazawa et al., Mar. 8, 1994. Some cast alloy wheels possess an ornamental design feature that provides for a smooth, stepless, radially outwardly curving, relatively thick appearing outboard flange. This appearance feature has proved to be popular in the aftermarket. An example of a wheel of this type is U.S. Pat. No. Des. 389,446, entitled xe2x80x9cRim for a Vehicle Wheelxe2x80x9d issued to Bradley, Jan. 20, 1998.
That is, cast alloy wheels, although economical to manufacture and appropriately decorative, possess certain drawbacks that render them undesirable to xe2x80x9cautomotive enthusiastsxe2x80x9d who desire wheels of lighter weight and greater strength. The major drawback of a cast alloy wheel is excess weight. The most common alloys (A356 and A357) used in the production of these wheels possess relatively low yield and ultimate tensile strengths. The casting process produces a metal structure that is porous and brittle in nature, therefore resulting in a lower fatigue life in comparison to cold rolled alloy sheet of equivalent thickness. To account for these deficiencies, manufacturers of cast alloy wheels must use a relatively large mass of material and thereby produce a wheel that is heavier than that produced by other methods. Quality control is also a problem in the manufacture of cast wheels as differential cooling between relatively thick and comparatively thinner sections of an integral one-piece wheel often results in the formation of voids and blowholes in the finished product.
Another method of producing automotive wheels is cold forging. Due to their high cost, cold forged wheels are rarely seen on passenger cars but, rather, are used on high performance racing vehicles almost exclusively. Cold forged wheels are manufactured from cold rolled aluminum alloy tube, bar, or plate stock. Cold rolled aluminum alloy has a dense grain structure, giving the metal excellent ductility and a high fatigue strength. Alloys commonly used in forging such as 7075 and 7050 series aluminum have significantly higher yield and tensile strength than the A356 and A357 casting alloys. A cold forged wheel may be manufactured as a one- or two-piece unit. The process begins by hammer forging the initial stock (bar, tube, or plate) into a donut shape. The rim portion is coaxially extruded from the forged blank. In the case of a one-piece unit, after extrusion of the rim, the hub is forged radially inwardly to its desired configuration. Critical surfaces are then finish machined. An example of a cold forged wheel is described in U.S. Pat. No. 5,446,962, entitled xe2x80x9cProcess of Manufacturing One Piece Forged Wheelsxe2x80x9d issued to Matossian et al., Sep. 5, 1995. Cold forged wheels have the characteristics of light weight and high strength that are greatly prized among xe2x80x9cautomotive enthusiastsxe2x80x9d. However, due to the high cost of producing these wheels, few enthusiasts can aspire to owning a set. Thus, there remains a need for a lightweight, high strength wheel having, as an ornamental design feature, an outboard flange that flares radially outwardly in an attractive manner and exhibits an appearance of having axial depth, that can be produced at a cost comparable to that of a cast wheel.
The present invention provides an improved cold formed (spun) alloy wheel rim which possesses the light weight and high strength characteristics which are prized by xe2x80x9cautomotive enthusiastsxe2x80x9d and are typically associated with forged wheels, at a cost comparatively less than that of a forged wheel. A further advantage of the invention is that it incorporates in a spun alloy rim a smooth, stepless, radially outwardly curving exterior flange which provides for a clean exterior rim appearance often associated with cast wheels. This clean, stepless, radially outwardly curving exterior flange is an ornamental design feature desired by xe2x80x9cautomotive enthusiastsxe2x80x9d that formerly was only available in cast alloy wheels.
The improved wheel rim of the present invention achieves its advantages through a novel combination of cold rolling and spinning manufacturing techniques. By means of this process, the improved rim""s exterior flange is rolled radially outwardly and turned back axially inwardly and then rolled radially inwardly to form an annular bearing surface defining the outboard wall of the bead seat, thereby providing for the smooth, stepless, radially outwardly curving exterior desired by enthusiasts. Further, the drop well of the improved rim is located entirely inboard of the rim""s centroidal axis, a feature that allows the wheel-center to be mounted at or near the wheel centroid, minimizing bending moments that occur when the wheel-center is offset from the wheel centroid. Thus, a lighter, stronger wheel can be fabricated, thereby making the wheel more attractive to xe2x80x9cautomotive enthusiastsxe2x80x9d.
Other features and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate by way of example, the features of the invention.